The high voltage scanning transmission electron microscope designed and built in this laboratory with the assistance of the current NIH grant will be evaluated as a means for the study of thick biological specimens, for which purpose it should have important advantages over the conventional high voltage electron microscope. Theoretical analysis has shown that the specimn thicknesses which can be used while retaining the same resolution and image contrast should be considerably greater. Further theoretical predictions which will be tested include the possibility of optimization of image quality by use of appropriate values for th collector aperture angle and by the application of an energy filter set to cut off electrons having more (or less) than a certain energy loss. An environment stage will be built and tested for the examination of specimens contained in any desired atmosphere of gas at pressures up to or exceeding atmospheric. This will allow the possibility of observing biological specimens in environments closely resembling those of normal living systems.